1. Field of the Invention
The present invention relates to an ultrasonic probe and an ultrasonic diagnostic apparatus.
2. Description of the Related Art
In the field of a medical ultrasonic diagnostic apparatus and a nondestructive inspection instrument, an ultrasonic probe is used for imaging the inner state of an object. The ultrasonic probe transmits ultrasonic waves toward the object and receives reflected echoes from an interface at which acoustic impedance changes in the object. In particular, the ultrasonic probe used in the medical ultrasonic diagnostic apparatus is formed in an array in which many oblong-shaped piezoelectric transducers are arranged, making it possible to acquire a real-time tomogram image with high resolution by electronically controlling the ultrasonic beam.
A typical ultrasonic probe comprises a piezoelectric element including a piezoelectric material and electrodes formed on both surfaces thereof, a backing material formed on the back face of the piezoelectric element, and an acoustic matching layer formed on the front face of the piezoelectric element, and has a structure that the piezoelectric element and the acoustic matching layer are processed into an array. In general, an acoustic lens is formed on the acoustic matching layer. The electrodes formed on both surfaces of the piezoelectric material are connected to a flexible printed circuit board (FPC) and are connected further to a diagnostic apparatus.
The piezoelectric element is used as a transmitter receiver for the ultrasonic waves. The backing material is used for absorbing undesired ultrasonic waves radiated backward from the piezoelectric element. The acoustic matching layer is used for improving the efficiency of transmitting/receiving the ultrasonic waves by matching the acoustic impedances between the piezoelectric element and the human body. Therefore, the acoustic impedance of the acoustic matching layer is set to a range between that of the piezoelectric element (20 to 30 Mrayls) and that of the human body (1.5 Mrayls). In the case of using two or more acoustic matching layers, the acoustic impedances of the acoustic matching layers are set to be gradually diminished toward the human body. The reason why the acoustic matching layer is processed into an array together with the piezoelectric element is to suppress coupling between adjacent channels. The pitch of the array probe is set to approximately 0.1 to 0.2 mm in a smaller case. The acoustic lens plays a role of focusing the ultrasonic waves in transmitting/receiving.
An ultrasonic probe used for diagnosis of the heart or the liver in a human boy is required to have a resonance frequency of approximately 2 to 5 MHz. An ultrasonic probe used for diagnosis of, for example, the carotid artery positioned shallower than the heart and the liver is required to have a higher resonance frequency. In order to provide a higher resonance frequency, it is necessary to reduce the thickness of the piezoelectric material, which is in the vibrating direction. Further, in order to suppress undesired vibrations, it is necessary to set the width of the piezoelectric material in the array direction at 60% or less of the thickness.
Conventionally, lead zirconate titanate (PZT) piezoelectric ceramics has been used for the piezoelectric material, since it has a high electromechanical coupling coefficient k33′ of approximately 70% and permits a high conversion efficiency from electric signals to mechanical vibrations. Also, a piezoelectric material with a very high electromechanical coupling coefficient k33′ of approximately 80 such as Pb((Zn1/3Nb2/3)0.91Ti0.09)O3 piezoelectric single crystal made of a solid solution of lead zinc niobate and lead titanate has been developed in recent years, and application of the particular piezoelectric material to the ultrasonic probe is being studied.
The acoustic matching layer is formed on the front face of the piezoelectric element to improve transmitting/receiving efficiency of ultrasonic waves to the human body. An acoustic matching layer made of a material prepared by dispersing metal particles such as W particles in an organic resin has been known. In recent years, an acoustic matching layer made of a material prepared by dispersing zinc oxide particles in an organic resin has also been proposed (see Japanese Patent Disclosure No. 2004-104629).
However, if an ultrasonic probe is manufactured by using the above acoustic matching layer, such problems as follows would occur.
When the acoustic matching layer prepared by dispersing metal particles in an organic resin is used, it is necessary to cut the high-toughness metal hard to be cut. This brings about marked degradation of a cutting blade in array processing by dicing. Where the processing is continued using the degraded blade, chipping or crack is created in the piezoelectric material that is cut together with the acoustic matching layer. The chipping or crack created in the piezoelectric material gives rise to a nonuniform capacity of the piezoelectric element, which leads directly to the nonuniform sensitivity of the ultrasonic probe so as to lower the quality of a converted image.
On the other hand, in the case of the acoustic matching layer prepared by dispersing zinc oxide particles in an organic resin, it is necessary to increase the amount of the dispersed zinc oxide in order to provide desired acoustic impedance because the density of zinc oxide is low. Increase in the amount of the zinc oxide decreases the amount of the resin between the adjacent oxide particles so as to lower adhesion of the particles. Accordingly, the array processing by dicing causes significant shedding, resulting in a problem that highly-accurate fine processing cannot be achieved. Also, the acoustic matching layer may be used with covered by plated metal over the entire surface thereof. In this case, adhesion between the acoustic matching layer and the plated metal is so insufficient that the electrode may be stripped away during array processing by dicing. Since the electrode of the piezoelectric element on the side of the acoustic matching layer is connected to the ground plate via the electrode formed on the acoustic matching layer, loss of the electrode causes open-circuit defect. Further, it is possible that the open-circuit defect may be caused during use of the ultrasonic probe.